rplst8
New Member
Hi all, first post to these forums.
I have an SX-838 I picked up a while ago actually from ebay. When I first got it, it would play for about 30-45 minutes and then go into protect. It would cycle on and off and as time went on the cycles were quicker and quicker. I only paid like $150 for it, so I wasn't expecting much. I ended up just shelving it and figured I'd deal with it later as I got busy with other things.
So fast forward to now, I've gotten interested in this thing again and getting it working. When I plugged it in the other night it started doing the same thing right from the get go. So I grabbed my DMM and a service manual and started poking around.
First I checked the bias and offset. The left channel checks out OK. I tightened them up a little but it was within spec as is. Then I checked the right channel. I'm not sure which is offset and which is bias but the service manual has you check pin 20 to ground on the power amp board and bring that to 0 mV (which I did at first). Then you check between pins 22 and 23 and bring that under 30 mV. So during this time, the amp clicks off again and voltage measured on pin 5 to gnd just goes nuts. It's all over the place. I stuck an old analog meter on it so I could watch it swing, and it was going about +30 to -10 or so.
As time went on, the swings became more frequent and erratic. During this, the left channel was totally stable.
I checked all the power supply rails that I could. The power supply rails marked +45 and -45 were like +43.5 and -43.5. The rail that is labeled 35.5 was like 34.5. The 13V and -13V rails were rock solid at 13V and -13V damn near on the dot.
Basically, all the voltages were very very close to what they should be, and pretty solid. I saw +/- 0.1V fluctuation at times but only on the unregulated supplies. I also checked the supply going to the phono preamp as I've heard that can cause problems if there is a short. Again 22V and 13V were rock solid.
I measured the bias and offset pots too. They seem to be good. As I turned them (with the amp off) I got smooth transitions throughout the rotation of the wiper.
When the amp is not in protect, both channels will pass good sounding audio. Just as it goes into protect, the right side lets out a pretty loud bzzzt, and then the protective circuitry mutes the outputs.
Any ideas where to look next?
Thanks,
RL
I have an SX-838 I picked up a while ago actually from ebay. When I first got it, it would play for about 30-45 minutes and then go into protect. It would cycle on and off and as time went on the cycles were quicker and quicker. I only paid like $150 for it, so I wasn't expecting much. I ended up just shelving it and figured I'd deal with it later as I got busy with other things.
So fast forward to now, I've gotten interested in this thing again and getting it working. When I plugged it in the other night it started doing the same thing right from the get go. So I grabbed my DMM and a service manual and started poking around.
First I checked the bias and offset. The left channel checks out OK. I tightened them up a little but it was within spec as is. Then I checked the right channel. I'm not sure which is offset and which is bias but the service manual has you check pin 20 to ground on the power amp board and bring that to 0 mV (which I did at first). Then you check between pins 22 and 23 and bring that under 30 mV. So during this time, the amp clicks off again and voltage measured on pin 5 to gnd just goes nuts. It's all over the place. I stuck an old analog meter on it so I could watch it swing, and it was going about +30 to -10 or so.
As time went on, the swings became more frequent and erratic. During this, the left channel was totally stable.
I checked all the power supply rails that I could. The power supply rails marked +45 and -45 were like +43.5 and -43.5. The rail that is labeled 35.5 was like 34.5. The 13V and -13V rails were rock solid at 13V and -13V damn near on the dot.
Basically, all the voltages were very very close to what they should be, and pretty solid. I saw +/- 0.1V fluctuation at times but only on the unregulated supplies. I also checked the supply going to the phono preamp as I've heard that can cause problems if there is a short. Again 22V and 13V were rock solid.
I measured the bias and offset pots too. They seem to be good. As I turned them (with the amp off) I got smooth transitions throughout the rotation of the wiper.
When the amp is not in protect, both channels will pass good sounding audio. Just as it goes into protect, the right side lets out a pretty loud bzzzt, and then the protective circuitry mutes the outputs.
Any ideas where to look next?
Thanks,
RL